Sulfur takes part in many biogeochemical reactions that affect the global carbon and oxygen cycles. On short time scales and in the absence of oxygen, many microbes use organic carbon to reduce sulfate to sulfide, which may then react with iron to form pyrite. On much longer time scales, the net addition of oxygen to the atmosphere through organic carbon burial promotes sulfide oxidation on land and increases the amount of sulfate carried by rivers to the oceans. Two reports in this issue, by Wortmann and Paytan (1) on page 334 and Halevy et al. (2) on page 331, show that on time scales of millions of years, changes in the formation and dissolution of evaporite minerals can strongly impact marine chemistry and the carbon cycle (see the figure). On longer time scales, the sulfur cycle played a much greater role in regulating atmospheric oxygen levels than previously recognized.